Steam Family Sharing is out

Sharing is caring, and there’s no better way to show your friends and family you care than to let them have a taste of your abundant Steam library. Today, after six month in beta, Valve announced Steam Family Sharing is available to all Steam users.

http://www.pcgamer.com/2014/02/28/steam-...-released/

This is great. I've been into this since Beta, and it's really awesome.
The only bad side is that the other person mustn't be signed in. But otherwise... FREE GAAAAAAAAAAAAAAAAAAAAAAMES

Anyone know if this doesn't work when you're signed into mobile but not an actual computer?